What is a triathlon bike split calculator?

It estimates the bike leg from distance, FTP, target intensity factor, course profile, and setup, then reports target watts, speed, split, and TSS.

What intensity factor should I use for Ironman?

Many age-group athletes plan roughly 0.68-0.73 IF for full-distance racing, then adjust for heat, hills, and run durability.

What intensity factor should I use for 70.3?

Many 70.3 plans sit around 0.75-0.82 IF, but the right value depends on fitness, course difficulty, and how well you can run afterward.

Is this a physics simulator?

No. It is a practical estimate using broad modifiers. For course-specific optimization, use a dedicated course-file model.

Why does target intensity factor drop as the race gets longer?

Because carbohydrate supply becomes the limiter. The share of energy coming from carbohydrate rises steeply with intensity, your body stores only about 400-500g of it, and the gut can absorb roughly 90g per hour at best using mixed glucose and fructose. At Ironman distance an IF above about 0.73 burns fuel faster than you can replace it, which is why the deficit shows up as a walked marathon rather than as a slow bike split.

What is a normal bike TSS for a triathlon?

A well-paced Ironman bike leg typically lands around 270-290 TSS and a 70.3 around 170-190. Because TSS uses IF squared, stress climbs much faster than power: moving a 70.3 from 0.75 to 0.85 IF raises power 13% but stress 28%. Figures well above those ranges are a warning that the run has already been spent, not a sign of a strong ride.

Does body weight change the estimate?

It changes W/kg, which the speed estimate uses, but the model does not account for the fact that aerodynamic drag scales with frontal area rather than mass. In reality a larger powerful rider goes faster on flat roads than W/kg alone predicts, and a lighter rider does relatively better when climbing. If you are far from average proportions, trust the watts, W/kg, and TSS outputs more than the speed figure.

Power-based bike pacing

Triathlon Bike Split Calculator

Estimate a realistic race bike split from FTP, intensity factor, W/kg, course profile, and setup before you commit to the whole race plan.

Race distance

Bike distance

FTP

Body mass

Intensity factor

Course profile

Bike setup

Controlled 70.3 effort

02:57:38

Projected bike split

Watts

187 W

Speed

30.4 km/h

W/kg

2.49

TSS

180

Bike split checks

Speed is estimated from broad modifiers, not a course-file physics simulation.

Connect this to the race plan

Full race plannerBuild a complete Ironman or 70.3 plan from pace to nutrition.Run off the bikeCheck whether this bike effort protects your run split.Cutoff bufferSee whether the bike split leaves enough course-closure buffer.Fueling planTurn bike duration into bottles, gels, carbs, and sodium targets.

Intensity factor is the number that decides your race

Intensity factor is simply your target power divided by your FTP. An IF of 0.70 means riding at 70% of the power you could hold for an hour. It sounds like an abstract number, but it is the single most consequential decision in long-course triathlon, because it sets both your bike split and the condition of your legs at T2.

The reason IF drops as distance grows is glycogen. Your body stores roughly 400-500g of carbohydrate, and the proportion of energy coming from carbohydrate rather than fat climbs steeply with intensity. Ride an Ironman at 0.80 IF and you will burn through your stores well before the marathon, no matter how much you eat on the bike — there is a ceiling on how fast the gut can absorb carbohydrate, generally put at around 90g per hour with mixed glucose and fructose.

This is why the recommended IF ranges are not arbitrary conservatism. They are the intensities at which your fuel supply and your fuel demand roughly balance for the duration involved.

Target intensity factor by race distance

Default IF values used by this calculator, with the practical range most age-group athletes should plan within.

Target intensity factor by race distance
Distance	Bike distance	Default IF	Typical range	Why
Sprint	20 km	0.88	0.85-0.92	Short enough that glycogen is not limiting; the 5K run tolerates fatigue
Olympic	40 km	0.84	0.80-0.88	Roughly an hour of riding; the 10K still punishes real overreach
70.3	90 km	0.78	0.75-0.82	Half marathon off the bike makes fuel and durability the limiter
Ironman	180 km	0.70	0.68-0.73	Marathon off the bike; carbohydrate supply is the hard ceiling

Move toward the lower end of each range if the course is hilly or hot, if your longest ride is well short of race distance, or if your fueling plan is untested.

Course and setup speed modifiers

The calculator estimates a neutral speed from your W/kg and target watts, then applies these two multipliers. Both are published so you can judge whether they fit your race.

Course and setup speed modifiers
Type	Setting	Modifier	Effect on speed
Course	Fast / smooth	1.05x	About 5% faster
Course	Flat	1.00x	Baseline
Course	Rolling	0.94x	About 6% slower
Course	Technical	0.90x	About 10% slower
Course	Hilly	0.88x	About 12% slower
Setup	Road bike	0.93x	About 7% slower
Setup	Road bike + aero bars	0.97x	About 3% slower
Setup	Tri bike (baseline)	1.00x	Baseline
Setup	Optimised aero setup	1.04x	About 4% faster

Modifiers multiply together. A hilly course on a road bike gives 0.88 x 0.93 = 0.82, about 18% slower than a tri bike on flat roads at identical power.

Reading TSS as a warning light

The calculator reports Training Stress Score using the standard formula: hours × IF² × 100. Because IF is squared, stress rises much faster than intensity. Raising a 70.3 bike from 0.75 to 0.85 IF is a 13% increase in power but a 28% increase in accumulated stress.

As a rough guide, a well-paced Ironman bike lands somewhere around 270-290 TSS and a 70.3 around 170-190. Numbers meaningfully above those ranges do not mean you rode well; they mean you spent the run. Treat a high TSS as a prompt to lower the IF rather than as a target to hit.

The honest test of any bike plan is what happens next. Take the split this page produces into the run-off-the-bike calculator and check whether the run it implies is one you would be happy with.

What this calculator does not model

Speed here is solved from a power balance: your target watts against aerodynamic drag and rolling resistance on flat ground, with the course and setup modifiers applied afterwards. That is genuine physics for the dominant forces, but it is not a course simulation. There is no wind vector, no gradient-by-gradient integration over a real route file, and no measured drag area — your CdA is estimated from body mass on the assumption of average proportions in a standard tri position.

The practical consequence is that the estimate is most reliable for riders of typical build in a normal aero position on rolling-to-flat terrain, and least reliable if your position is unusually good or unusually poor. If you have measured your own CdA, you already have better data than this page can infer.

For genuine course optimisation — wind vectors, gradient-specific power targets, equipment trade-offs — use a dedicated course-file model such as BestBikeSplit. This page is for deciding whether your intensity target is sane before you get that far.

How to use the result

Start with a recent FTP, choose a race distance, and keep intensity factor conservative enough that the run still exists. The calculator turns the power target into watts, W/kg, estimated speed, split time, and bike TSS.

Use lower IF for Ironman than 70.3 or Olympic racing.
Treat hilly, windy, or technical courses as run-risk multipliers.
Use TSS as a warning, not a medal for riding hard.

Example race read

A 220 W FTP athlete riding a 70.3 at 0.78 IF targets about 172 W. If the course is rolling and the setup is average, the projected split should be checked against the run-off-bike calculator before calling the plan aggressive or realistic.

Common mistakes

The most expensive mistake is chasing a bike split in isolation. A fast ride that creates high TSS, missed fueling, or a heavy first 5 km of the run is not a good triathlon split. The right bike target protects total finish time.

Next best calculators

Run Off The BikeEstimate the run cost of the bike target before race day.Course AdjustmentApply terrain and course friction to the split estimate.Goal Split PlannerConnect bike watts back into the full swim-bike-run target.

Frequently asked questions

What is a triathlon bike split calculator?: It estimates the bike leg from distance, FTP, target intensity factor, course profile, and setup, then reports target watts, speed, split, and TSS.
What intensity factor should I use for Ironman?: Many age-group athletes plan roughly 0.68-0.73 IF for full-distance racing, then adjust for heat, hills, and run durability.
What intensity factor should I use for 70.3?: Many 70.3 plans sit around 0.75-0.82 IF, but the right value depends on fitness, course difficulty, and how well you can run afterward.
Is this a physics simulator?: No. It is a practical estimate using broad modifiers. For course-specific optimization, use a dedicated course-file model.
Why does target intensity factor drop as the race gets longer?: Because carbohydrate supply becomes the limiter. The share of energy coming from carbohydrate rises steeply with intensity, your body stores only about 400-500g of it, and the gut can absorb roughly 90g per hour at best using mixed glucose and fructose. At Ironman distance an IF above about 0.73 burns fuel faster than you can replace it, which is why the deficit shows up as a walked marathon rather than as a slow bike split.
What is a normal bike TSS for a triathlon?: A well-paced Ironman bike leg typically lands around 270-290 TSS and a 70.3 around 170-190. Because TSS uses IF squared, stress climbs much faster than power: moving a 70.3 from 0.75 to 0.85 IF raises power 13% but stress 28%. Figures well above those ranges are a warning that the run has already been spent, not a sign of a strong ride.
Does body weight change the estimate?: It changes W/kg, which the speed estimate uses, but the model does not account for the fact that aerodynamic drag scales with frontal area rather than mass. In reality a larger powerful rider goes faster on flat roads than W/kg alone predicts, and a lighter rider does relatively better when climbing. If you are far from average proportions, trust the watts, W/kg, and TSS outputs more than the speed figure.

Trust and methodology

How this page should be used

Last updated

August 8, 2026

Maintained by

M Imtinan Farooq

Status

Planning estimate, not a race guarantee

Formula summary

Target watts = FTP x IF; TSS = hours x IF squared x 100; speed solves power against aerodynamic drag and rolling resistance, then applies course and setup modifiers.

Key assumptions

FTP, body mass, course profile, and setup are realistic. Drag area is estimated from body mass rather than measured.

Limitations

Not a GPS course-file model: no wind vector, gradient-by-gradient integration, or measured CdA.

Full formulas, source notes, and limitation details are maintained on the methodology page. Use official race guides for event rules, cutoff times, venue policies, and safety instructions.